A nephelometer[1] or aerosol photometer[2] is an instrument for measuring the concentration of suspended particulates in a liquid or gas colloid.

This information helps understand the character of particulate pollution inside a building or in the ambient air, as well as the cleanliness level in a controlled environment.

[citation needed] Biological contaminants include mold, fungus, bacteria, viruses, animal dander, dust mites, pollen, human skin cells, cockroach parts, or anything alive or living at one time.

Three wavelength nephelometers fitted with a backscatter shutter can determine the amount of solar radiation that is reflected back into space by dust and particulate matter.

This reflected light influences the amount of radiation reaching the earth's lower atmosphere and warming the planet.

Nephelometers, through the measurement of light scattering, can determine visibility in distance through the application of a conversion factor called Koschmieder's formula.

These have the capability to detect extremely low particle concentrations (to 0.005%) and are therefore highly suitable to protecting sensitive or valuable electronic equipment, such as mainframe computers and telephone switches.